1. Field of the Invention
The present invention relates to a zoom lens that is suitably used in a video camera, a film camera, a broadcasting camera, a digital still camera and the like and to an image pickup apparatus equipped with such a zoom lens.
2. Description of the Related Art
In recent years, size reductions and improvements in functionality have been achieved in image pickup apparatuses using a solid state image pickup element such as video cameras, digital still cameras, broadcasting cameras and still cameras using a film.
Along with this, it is desired that a taking optical system used in such an image pickup apparatus is required to be a compact zoom lens that is short in the entire length and has a high zoom ratio.
A compact, high zoom-ratio zoom lens composed of four lens units respectively having a positive refractive power, negative refractive power, positive refractive power and positive refractive power in the mentioned order from the object side to the image side has been known (see Japanese Patent Application Laid-Open No. H03-296706, U.S. Pat. No. 6,975,461 and Japanese Patent Application Laid-Open No. 2004-94233). In this zoom lens, zooming is effected by moving the lens units.
In a known four-lens-unit zoom lens having the above described configuration, the second lens unit is composed of three negative lenses and one positive lens (see Japanese Patent Application Laid-Open No. H07-177926). In another known four-lens-unit zoom lens, the second lens unit is composed of five lenses (see U.S. Pat. No. 6,462,886 and Japanese Patent Application Laid-Open No. 2000-368338).
In recent years, image pickup apparatuses have been made smaller, and accordingly a reduction in the size of the zoom lens is highly desired. In particular, zoom lenses used in image pickup apparatuses are required to be made smaller in the size of the entire lens system while having a predetermined zoom ratio and having satisfactory optical performance throughout the zoom range.
Generally speaking, the length of each lens unit that constitutes a zoom lens along the optical axis increases with an increase in the number of the lenses in the lens unit.
In addition, when the movement amount of each lens unit in zooming and focusing is large, the entire length of the zoom lens becomes long, which makes it difficult to make the lens small.
To reduce the size of a zoom lens and to increase the zoom ratio thereof, it is necessary to appropriately design movement conditions of each lens unit in zooming, the refractive power of each lens unit, and the lens configuration of each lens unit. For example, when it comes to reduction in the size of a zoom lens, increasing the refractive power of each lens unit leads to a decrease in the movement amount of each lens unit, whereby the entire length of the lens system can be made smaller. However, if the refractive power of each lens unit is simply increased, variations in aberrations upon zooming become so large that it becomes difficult to correct the aberrations satisfactorily.
As will be seen from the above, size reduction and excellent optical performance are generally contrary to each other, and it is a significant challenge in designing a zoom lens to achieve both size reduction and excellent optical performance.